Select Settings in the top left banner.
Select the User Portal tab.
Scroll down the page to the Self-Registration section and check the Enable Self-Registration box to open additional functions.
Within the Self-Registration section in the End user domains allowed to register field you will be able to restrict end users to only be able to register with specific domains.
If you do not want to restrict the end users to specific domain leave the End user domains allowed to register field blank.